The present invention relates to an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus and to a developing agent used in the image forming apparatus, particularly, to an image forming apparatus in which the development is performed by a magnet rotation developing system and a two-component developing agent used in the image forming apparatus, said developing agent containing a magnetic toner and a carrier.
In an image forming apparatus using a magnet rotation developing system, the developing roller is arranged to face an image carrier bearing an electrostatic latent image. The developing roller comprises, for example, a hollow cylindrical sleeve made of a magnetic material and arranged rotatable and a magnetic roll arranged within the sleeve, consisting of a plurality magnetic poles and rotatable independently of the sleeve.
In the magnet rotation developing system, a two-component developing agent consisting of a magnetic carrier and a magnetic toner having a magnetic material added to the surface of a magnetic toner particle is applied to the developing roll. Also, the magnetic roll and the sleeve are rotated in the same direction or opposite direction so as to transfer the developing agent into the developing region while rotating the developing agent about its own axis. In such a magnet rotation developing system, the specific concentration of the toner can be controlled easily, compared with the ordinary magnet fixation developing system. Therefore, the specific concentration of the toner can be increased in the magnet rotation developing system. It follows that the toner transfer amount is increased so as to improve the developing efficiency.
Where the magnet roll and the sleeve are rotated in the same direction, the rotating direction of the developing agent about its own axis is opposite to the transfer direction of the developing agent. On the other hand, where the magnet roll and the sleeve are rotated in the opposite directions, the rotating direction of the developing agent about its own axis is equal to the transfer direction of the developing agent so as to increase the transfer amount of the developing agent. Therefore, the rotation of the magnet roll and the sleeve in the opposite directions is adapted to the development at a high speed.
In general, in the two-component development of the magnet fixation developing system, the specific concentration of the toner on the developing roll is about 6% by weight. In the two-component development of the magnet rotation developing system, however, the specific concentration of the toner on the developing roll is maintained at about 50% by weight, leading to a merit that a so-called xe2x80x9ccarrier attachmentxe2x80x9d, i.e., the phenomenon that the carrier on the developing roll is attached to the image carrier, is less likely to take place.
A developing agent that can be used in the two-component development of such a magnet rotation developing system is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 3-256052. Disclosed in this prior art is a developing agent containing a magnetic toner particle, a magnetic powder added to the magnetic particle, and a carrier. It is specified that the product among the true specific gravity, the average particle diameter and the magnetization of the carrier is greater than the product among the true specific gravity, the average particle diameter and the magnetization of the magnetic toner.
In the developing agent disclosed in this prior art, however, the surface of the photoreceptor is unduly scraped off by the magnetic powder mixed and attached to the magnetic toner particle. As a result, the sensitivity of the photoreceptor is deteriorated, resulting in failure to provide a stable charging and in a shortened life of the photoreceptor.
On the other hand, in the case of using a toner having such a magnetic powder removed therefrom, the conventional magnetic carrier fails to be charged sufficiently under a high temperature and a high humidity, resulting in decrease in the image concentration. Also, in the case of using a magnetic carrier having a high charging capability, the charging capability of the toner is rendered unduly high, resulting in an inconvenience that the image density is lowered in the case where the printing is carried out continuously. Further, in the case of using a magnetic carrier having a high charging capability, the margins in terms of the electrical resistance and the magnetic force are small, making the optimization difficult.
A first object of the present invention, which has been achieved in view of the situation described above, is to provide a developing agent, which permits adjusting the scraping of the image carrier caused by the presence of the magnetic powder, which is satisfactory in its cleaning properties, and which permits forming a satisfactory image free from blurring and fogging through the entire life of the developing agent.
A second object of the present invention is to provide an image forming apparatus, which permits adjusting the scraping of the image carrier caused by the presence of the magnetic powder, which is satisfactory in its cleaning properties, and which permits forming a satisfactory image free from blurring and fogging through the entire life of the developing agent.
Further, a third object of the present invention is provide an image forming method, which permits adjusting the scraping of the image carrier caused by the presence of the magnetic powder, which is satisfactory in its cleaning properties, and which permits obtaining a satisfactory image free from blurring and fogging through the entire life of the developing agent.
According to a first aspect of the present invention, there is provided a developing agent comprising toner particles containing a first magnetic powder and a binder resin, 0.5 to 5% by weight based on the total weight of the toner particles of a second magnetic powder, and 0.01 to 0.5% by weight based on the total weight of the toner particles of stearate particles having an average particle diameter of 1 xcexcm to 5 xcexcm.
According to a second aspect of the present invention, there is provided an image forming apparatus, comprising:
at least one image carrier;
a developing device housing a toner having toner particles containing a first magnetic powder and a binder resin, 0.5 to 5% by weight based on the total weight of the toner particles of a second magnetic powder, and 0.01 to 0.5% by weight based on the total weight of the toner particles of stearate particles having an average particle diameter of 1 xcexcm to 5 xcexcm, including a developing roller having a hollow cylindrical sleeve arranged rotatable and a magnet roll arranged within the sleeve, rotatable independently of the sleeve and having a plurality of magnetic poles, and serving to develop an electrostatic latent image formed on the image carrier so as to form a developing agent image;
a transfer device for transferring the developing agent image;
a cleaning device for cleaning the developing agent that was not transferred; and
a fixing device arranged below the transfer device for fixing the transferred developing agent image.
According to a third aspect of the present invention, there is provided an image forming method, comprising the steps of:
supplying a developing agent containing toner particles having a first magnetic powder and a binder, 0.5 to 5% by weight based on the total weight of the toner particles of a second magnetic powder and 0.01 to 0.5% based on the total weight of the toner particles of stearate particles having an average particle diameter of 1 xcexcm to 5 xcexcm onto a developing roller arranged to face an image carrier and having a hollow cylindrical sleeve rotatable in synchronism with the image carrier and a magnet roll having a plurality of magnetic poles and rotated independently of the sleeve so as to develop an electrostatic latent image formed on the image carrier to form a developing agent image;
transferring the developing agent image onto a transfer material; and
fixing the transferred developing agent image.
According to the present invention, it is possible to form an image satisfactory in cleaning properties, free from blurring of image and fogging over the entire life of the image, and high in image density and a resolution.